Generally, a vinyl chloride-based resin (hereinafter may be referred to as a vinyl chloride-based polymer) is produced by suspension polymerization in which a vinyl chloride monomer is dispersed in an aqueous medium with a polymerization initiator, a dispersion stabilizer, etc. for polymerization.
Dispersants used in the suspension polymerization are divided into so-called “dispersion stabilizers” added to stabilize the dispersibility of the vinyl chloride monomer and adjust the particle diameter of the vinyl chloride-based resin to be produced, and so-called “dispersion assistants” added to increase the porosity of the vinyl chloride-based resin particles to be produced.
Conventionally, as the “dispersion stabilizer” (hereinafter may be referred to as primary dispersant), polyvinyl alcohol (PVA), hydroxypropylmethylcellulose, etc. are used (see Patent Literature 1 to 3). As the “dispersion assistant”, PVAs having a saponification degree lower than that of the primary dispersant are used (see Patent Literature 4).
For example, in the method for producing a vinyl chloride-based polymer described in Patent Literature 4, a PVA having a saponification degree of 75 to 85 mol % is used as a dispersion stabilizer, and a PVA having a saponification degree of 20 to 57 mol % is used as a dispersion assistant.
For increased performance of the dispersion assistant, e.g., for an increased porosity of the particles of the vinyl chloride-based resin, other various suggestions have been made as in Patent Literature 5 to 7.
However, there has never been a dispersion assistant for suspension polymerization so far that achieves, regardless of the polymerization conditions, stable production of a good polymer without much scale adhesion to the polymerization tank even when used in a large amount to obtain a high porosity, and that enables production of a vinyl chloride-based resin having a high porosity and an excellent plasticizer absorbability, not having coarse particles. In particular, to obtain a vinyl chloride-based resin having a very high porosity, an increased amount of a dispersion assistant is needed, but use of such an increased amount causes a problem of unstable polymerization.